1. Field of the Invention
The present invention relates to a power supply, and more particularly to a power supply with output protection and a control method of the power supply.
2. Description of Related Art
With reference to FIG. 4, a circuit diagram of a conventional power supply is disclosed. The conventional power supply has a rectifier circuit 81, a PFC (Power Factor Correction) circuit 82, a transformer 83, an output circuit 84, a power switch device 85, a PWM (Pulse Width Modulation) controller 86, a feedback circuit 87 and a microcontroller 88.
An input terminal of the output circuit 84 is connected to a secondary side of the transformer 83. An output terminal of the output circuit 84 is connected to a load, i.e. a server system. The power switch device 85 is connected to a primary side of the transformer 83 and is controlled by the PWM controller 86 to activate the output circuit 84 for providing the load with a working voltage or stopping outputting the working voltage. The PWM controller 86 is connected to the output terminal of the output circuit 84 via the feedback circuit 87 to acquire an output voltage from the output circuit 84.
The microcontroller 88 has an output current detection terminal (Cs1), an alarm signal output terminal (IO1) and an output terminal. The output current detection terminal (Cs1) is connected to the output terminal of the output circuit 84 through a differential amplifier 89 to acquire an output current from the output circuit 84. The output circuit 84 has an output resistor (Rs1). Two terminals of the output resistor (Rs1) are connected to two input terminals of the differential amplifier 89. An output terminal of the differential amplifier 89 is connected to the output current detection terminal (Cs1) of the microcontroller 88. The microcontroller 88 can acquire the output current via the differential amplifier 89. The alarm signal output terminal (IO1) and the output terminal of the microcontroller 88 are respectively connected to the load and the PWM controller 86.
The conventional power supply has an output protection function, usually an over-current protection function. The microcontroller 88 acquires a present output current from the output terminal of the output circuit 84 through the differential amplifier 89. When the present output current is higher than a default ratio of a rated current, for example 125%, the present output current reaches an over-current protection point. The microcontroller 88 then turns off the PWM controller 86, such that the output circuit 84 stops providing the working voltage to the load. Meanwhile, the microcontroller 88 informs the load through the alarm signal output terminal (IO1). Only when a current received by the load from the power supply is lower than the over-current protection point, the power supply is rebooted to restore the working voltage.
Although the power supply is protected from over-current, for a manufacturer, the over-current protection point should be accurately and carefully designed. The power supply and the load will be easily damaged due to the over-current condition if the over-current protection point is too high. On the other hand, if the over-current protection point is too low, the over-current protection function will not effectively protect the power supply and the load.
When the power supply works in a steady state, the output current will not be higher than the over-current protection point. However, the output current may approximate or be higher than the over-current protection point only when the power supply is booting. A user has to choose an expensive high-performance power supply to prevent a high booting current from accidently triggering the protection action.
In brief, a main purpose of the high-performance power supply is only to restrain the temporarily booting current. When the current is stable, the high-performance power supply acts as a general power supply. The high-performance power supply is usually much expensive than a general one. The cost for building a power system with the high-performance power supply is high.